1. Field of the Invention
This invention relates to improvements in a solid electrochromic element.
2. Description of the Prior Art
The conventional solid electrochromic element has comprised a solid electrochromic substance layer and a dielectric material layer capable of causing an oxidation-reduction reaction, said two layers being laminated between two electrodes. This element has been such that when a DC field is applied between the two electrodes, an oxidation-reduction reaction is caused between said two layers to cause coloration of the electrochromic layer. However, in this conventional element, a great leakage current which does not directly contribute to the coloration flows between the two electrodes and this has led to a disadvantage that the power consumed is great.
A technique for overcoming such disadvantage is disclosed in Japanese Laid-Open Patent Publication No. 73749/1977. The electrochromic element disclosed in this publication will now be described with reference to FIG. 1 of the accompanying drawings.
The conventional element of FIG. 1 comprises one transparent electrode 2a laminated adjacent to a glass substrate 1, a dielectric material layer 3 composed of chromium oxide (Cr.sub.2 O.sub.3) laminated adjacent thereto, an electron blocking layer 4 composed of tantalum oxide (Ta.sub.2 O.sub.5) laminated adjacent thereto, a solid electrochromic substance layer 5 composed of tungsten trioxide (WO.sub.3) laminated adjacent thereto, and the other transparent electrode 2b laminated adjacent thereto.
The electron blocking layer 4 of course blocks the movement of electrons. The electron blocking layer 4 is considered to permit the movement of ions in order for the two layers 3 and 5 to cause an oxidation-reduction reaction therebetween.
The operation of this element will hereinafter be described.
When a main switch is closed and the electrodes 2a and 2b are connected to the positive pole and the negative pole, respectively, of a DC power source S, the solid electrochromic substance layer 5 and the dielectric material layer 3 cause an oxidation-reduction reaction therebetween and the electrochromic substance layer 5 produces a color. The electron blocking layer 4 blocks the electrons moving between the electrodes which do not contribute to the oxidation-reduction reaction, namely, the leakage current.
In the conventional element as described hereinbefore, WO.sub.3, MoO.sub.3, etc. used for the solid electrochromic substance layer are oxides which are usually liable to become an N-type semiconductor depending on the manufacturing conditions. On the other hand, substances such as Cr.sub.2 O.sub.3 and the like used for the dielectric material layer are ones which are liable to become a P-type semiconductor depending on the manufacturing conditions. Accordingly, when a voltage of the coloring polarity is applied to a solid electrochromic element comprising a solid electrochromic substance layer which is an N-type semiconductor and a dielectric material layer which is a P-type semiconductor, just the same effect as that attained by applying a forward voltage to the P-N junction is obtained and if the applied voltage becomes greater, a leakage current will flow even if there is an electron blocking layer. Such leakage current will become greater as the applied voltage becomes greater.